Image Processing Apparatus for Correcting Distortion of Image and Image Shooting Apparatus for Correcting Distortion of Shot Image

1. An image processing apparatus which corrects distortion of an image, and comprises: a shot-image acquisition section which acquires a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot, wherein a real shape of said subject to be shot is a rectangle; a reduced-image acquisition section which acquires a reduced image that is said shot image with a reduced resolution; a distortion information acquisition section which acquires a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting, acquires a contour of said shooting target image based on said corrected reduced image, specifies four line portions of said contour, and acquires a shape of a trapezoid formed by said four line portions as distortion information; a projection parameter acquisition section which acquires a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and a projection transformation section which produces a projection-transformed image by performing projection transformation on said shot image using said projection parameter.

2. The image processing apparatus according to claim 1 , wherein said line portions are specified based on Hough transformation, and lines corresponding to a top side, a bottom side, a left side and a right side of said trapezoid are specified according to which directions to a center of said corrected reduced image angles of said line portions acquired by said Hough transformation correspond.

3. The image processing apparatus according to claim 1 , wherein said projection transformation section performs said projection transformation at a same time as extracting said shooting target image from said shot image.

4. The image processing apparatus according to claim 1 , wherein said projection transformation section performs said projection transformation in association with each pixel position constituting said projection-transformed image, specifies a position on said shot image which corresponds to said each pixel position on said projection-transformed image based on said projection parameter, and uses a pixel value on said shot image which corresponds to said specified position as a pixel value corresponding to said each pixel position on said projection-transformed image.

5. The image processing apparatus according to claim 4 , wherein said projection transformation section computes said pixel value corresponding to each pixel position on said projection-transformed image by performing pixel interpolation based on a plurality of pixels near a position on said shot image specified based on said projection parameter.

6. The image processing apparatus according to claim 4 , wherein a resolution of said projection-transformed image is lower than a resolution of said shot image.

7. The image processing apparatus according to claim 1 ,wherein said subject to be shot is a sheet, and said shooting target image is an image of an original on which what is written on said sheet is transferred.

8. The image processing apparatus according to claim 1 , wherein said image processing apparatus is an image shooting apparatus comprising the lens, and an image pickup section which shoots a subject to be shot through said lens, thereby acquiring a shot image, and wherein a shot image taken by said image pickup section is said shot image to be acquired by said shot-image acquisition section, and said projection-transformed image acquired by said projection transformation section is recorded.

9. An image processing apparatus which corrects distortion of an image, comprising: a shot-image acquisition section which acquires a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot; a reduced-image acquisition section which acquires a reduced image that is said shot image with a reduced resolution; a distortion information acquisition section which acquires a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting, and acquires distortion information indicative of linear distortion of said shooting target image based on said corrected reduced image; a projection parameter acquisition section which acquires a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and a projection transformation section which produces a projection-transformed image by performing distortion correction according to said characteristic of said lens concurrently with performing projection transformation on said shot image using said projection parameter.

10. An image shooting apparatus which corrects distortion of a shot image, comprising: a lens having a predetermined characteristic; an image pickup section which shoots a subject to be shot through said lens, thereby acquiring a shot image, wherein a real shape of said subject to be shot is a rectangle; a corrected reduced-image acquisition section which acquires a corrected reduced image by reducing a resolution of said shot image and performing distortion correction according to said characteristic of said lens; a distortion information acquisition section which acquires a contour of said shot subject based on said corrected reduced image, specifies four line portions of said contour, and acquires a shape of a trapezoid formed by said four line portions as distortion information; and a projection transformation section which converts said shot image to an ideal distortion-free image based on said distortion information.

11. The image shooting apparatus according to claim 10 , further comprising a display section which displays said shot image, and wherein said reduced-image acquisition section reduces said shot image in association with a display size of an image to be displayed by said display section.

12. The image shooting apparatus according to claim 10 , further comprising a recording section which records a projection-transformed image produced by said projection transformation section.

13. An image processing method comprising: acquiring a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot, wherein a real shape of said subject to be shot is a rectangle; acquiring a reduced image that is said shot image with a reduced resolution; acquiring a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting; acquiring a contour of said shooting target image based on said corrected reduced image, specifying four line portions of said contour, and acquiring a shape of a trapezoid formed by said four line portions as distortion information; acquiring a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and producing a projection-transformed image by performing projection transformation on said shot image using said projection parameter.

14. A recording medium having a program stored thereon which is executable by an image processing apparatus to execute functions comprising: acquiring a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot, wherein a real shape of said subject to be shot is a rectangle; acquiring a reduced image that is said shot image with a reduced resolution; acquiring a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting; acquiring a contour of said shooting target image based on said corrected reduced image, specifying four line portions of said contour, and acquiring a shape of a trapezoid formed by said four line portions as distortion information; acquiring a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and producing a projection transformed image by performing projection transformation on said shot image using said projection parameter.

15. A recording medium having a program stored thereon which is executable by an image processing apparatus to execute functions comprising: acquiring a shot image of a subject to be shot, including a shooting target image which is an image portion corresponding to said subject to be shot; acquiring a reduced image that is said shot image with a reduced resolution; acquiring a corrected reduced image by performing distortion correction on said reduced image according to a characteristic of a lens used in shooting; acquiring distortion information indicative of linear distortion of said shooting target image based on said reduced image; acquiring a projection parameter for converting said shooting target image to an ideal distortion-free image based on said distortion information; and producing a projection-transformed image by performing distortion correction according to said characteristic of said lens concurrently with performing projection transformation on said shot image using said projection parameter.